With the continuous expansion of global energy demand, large onshore oil and gas fields are increasingly exhausted. People are gradually turning their attention to the ocean. The marine oil and gas resources are mainly distributed on the continental shelf, accounting for about 60% of the global marine oil and gas resources. The potential of oil and gas resources in deep and ultra deep waters is considerable, accounting for about 30%. Low temperature, soft bottom and shallow layer flow are the common characteristics of this kind of strata.

The low-density cement slurry system which is suitable for surface cementing in deep water can solve the problems faced by surface cementing in deep water, and it can be applied to wells with a depth of more than 500 m in China, which is conducive to improving the overall level of deep-water oil and gas well cementing in China.

1. Development of deep water low density cement slurry system

1.1 technical background

According to the geophysical survey, the well will encounter faults and there is a risk of lost circulation. In addition, for the formation with the same sedimentary thickness, with the increase of water depth, the fracture pressure gradient of the formation is decreasing, resulting in a narrow window between the fracture pressure gradient and the formation pore pressure gradient, which is prone to lost circulation.

The commonly used low density cement slurry at home and abroad mainly includes floating beads, bentonite, fly ash and other systems. From the field experience, the density of these low density systems has the lowest limit value. When the density is lower than this limit value, the performance of cement slurry changes greatly, especially the stability of slurry and compressive strength of cement stone. For example, the minimum density limit value of floating bead low density cement slurry system is 1.4g/cm3; the minimum density limit value of low density cement slurry on bentonite is 1.6g/cm3; the minimum density limit value of fly ash low density cement slurry is 1.55g/cm3. When the density of each cement slurry is lower than this density, the cement slurry will be stratified and the compressive strength will drop rapidly.

1.2 selection of new low density lightening materials

The main reason for the poor pressure bearing capacity of the low-density floating bead system is that the density reducing material used is the by-product of coal combustion, with different particle size, irregular shape and inconsistent composition, resulting in its poor pressure bearing capacity. The hollow glass bead ZOC-P62S developed and improved by Zoranoc Oilfield Chemical Research Institute is a factory product with better performance Particle size distribution and strength.

ZOC-P62S artificial glass beads are mainly composed of alkali lime borosilicate glass. They are transparent micron glass sealed hollow positive spheres with hard spherical shell. The spheres are filled with thin gas. Macroscopically, it is pure white powder. The minimum particle size is 2 mm and the average particle size is 3.5 mm. The wall thickness is about 10% of its diameter. As a new type of inorganic filler, it has the characteristics of low density, strong compression resistance and excellent stability.

The results show that this series of hollow glass beads are perfect spheres with small particle size, easy to mix and pump; they are incompressible and can be used for logging conveniently and accurately; they have a high strength density ratio and will not be broken in downhole operation; they have a fairly high sealing rate, water can not enter the sphere, so the density can be kept constant; they are chemically inert and will not mix with cement slurry The results show that the addition agent can react with other additives, so that it can be compatible with all cementing slurry systems; the isotropic stress of micro beads is consistent, which can reduce the shrinkage of cement after solidification; there is a little gas inside, so it has a good heat preservation effect, which can speed up the hydration rate of cement, thus reducing the waiting time, and making the cement have a higher strength in a short time Degree.

In view of the narrow surface pressure window in deep water and ultra deep water cementing and the high water depth pressure on seabed cement slurry, ZOC-P62S with smaller particle size, lower density and higher compressive strength is selected as density reducing agent for deep water.

1.3 design principles of low density cement slurry system

According to the close packing theory of particle gradation, the strength and other properties of low-density cement slurry can be greatly improved by optimizing the particle size distribution in low-density mixture and improving the compactness of cement slurry system. Therefore, attention should be paid to the following issues:

1) Good stability: under certain conditions, the cement paste does not separate into layers, and the vertical density distribution of cement paste is basically consistent, with less free liquid and small volume shrinkage.

2) Suitable rheological property and density: the type and grade particle size of the lightening agent should be selected, and the water consumption should not be increased blindly. The water consumption should be strictly controlled within the maximum allowable range of the selected lightening agent.

3) Higher early strength: the rheological properties and filtration control of cement slurry should not be blindly pursued, which will damage the compressive strength and stability of cement slurry. Low density materials with different particle sizes should be selected for tight packing wells with additives to improve the early strength.

1.4 deep water surface cement slurry system and performance evaluation

According to the actual well condition requirements of LW, combined with the development of a new type of deep-water low-temperature admixture, a new type of ZOC-LET^TM LT deep-water low-temperature low-density cement slurry system is used

Jiahua grade G cement, artificial micro bead ZOC-P62S, reinforcing agent ZOC-BT1, deep water cementing accelerator ZOC-DA92S, ZOC-DA93L, fluid loss additive ZOC-DG72S, defoamer ZOC-X60L, dispersant ZOC-F44L, anti channeling enhancer ZOC-GS12L, field drilling water. A lot of formula optimization experiments have been done in the laboratory, and the cement slurry formula for 20 "casing cementing is obtained, and the performance of the cement slurry is evaluated in detail.

2. Field application

LW is located about 260km southeast of Hong Kong. Liuhua 28-4 structure is located in zhuii depression, southern depression of Pearl River Mouth Basin, and northern slope of dongwa in Baiyun sag. A large amount of oil and gas resources were discovered in py-lh reservoir forming area of Panyu Low Uplift in 2011. It is proved that the oil and gas accumulation conditions in this area are very favorable. At the same time, it also reveals the great potential of oil and gas exploration in Baiyun sag Sandstone in the lower part of Zhujiang Formation (2996.0m-3096.0m) and T62 sandstone (3554.0m-3616.0m) of Zhuhai Formation. The water depth is 561 M. three spud operations are carried out: 36 "pipe is put into the first spud; 26" hole is opened in the second for 20 "cementing operation; 17-1 / 4" hole is opened for 13-3 / 8 ", and no cementing operation is conducted for the fourth spud.

Field construction of 20 "casing:

The formation fracture pressure of 20 "shallow casing cementing is low, and there is a risk of cementing loss: the formation is soft, and the additional amount is difficult to determine. According to the existing difficulties, after the casing runs to the designed well depth, the flow rate starts to circulate with 30spm displacement, and gradually increases the displacement to 130 SPM after returning to normal; After circulating seawater for 2 weeks, the display fluid of cementing pump is l0bbl, and the flushing fluid is 60bbl. The cement slurry with mixed pump density of 1.35s. G needs 489 BBL of mixed water and 98T of floating bead cement; 571 BBL of cement slurry with 1.43S.G of mixed pump density is 269 BBL. 86T of floating bead cement; 92.53 BBL of seawater was replaced by cementing pump. No backflow was found during pressure relief inspection.

The waiting time is 10 hours (continuous operation on site, dropping drill pipe and running BOP). The position of probe plug is 1078.5m and the pressure is 20 klbs to confirm. Cobalt casing accessories and cement plug to 1100m, casing pressure test 300PSI / 5min, 1500psi / 15min, qualified 1100m, in line with the design. No cement block was found during the third spud drilling.

3. Conclusion

3.1 the artificial glass bead lightening agent developed by the oilfield business unit is more suitable for deep water and ultra deep water surface cementing operations than natural floating beads.

3.2 the oilfield chemical division independently developed the deep-water low-temperature surface cementing low-density cement slurry system with good comprehensive performance and stable system, which has been well applied in the South China Sea LW field.